Leaf Temperature and Sap Flow Dynamic Coupling in Canopy Dominant Trees in the Central Amazon

Abstract:

The Amazon forest is the largest intact tropical forest in the world with globally important fluxes of carbon, water and energy. In 2005, 2010 a strong el Nino occurred resulting in a large scale drought across the Amazon basin, with increases in the temperature, tree mortality, emissions of CO2 and water cycling. In order to improve our ability to predict terrestrial carbon and water cycling in the Amazon basin under future climate warming scenarios, a better understanding of interactions between plant physiology and hydrology is necessary. Although few measurements exists on tropical forests and it is generally assumed that a temporal lag exists between leaf temperature and stem sap flow. Here we show a tight temporal positive correlation (eg: R² = 0.80) between sap flow velocity and leaf temperature in canopy dominant trees in central Amazon forest during the 2015 el Nino. As the drought progressed in September 2015, further increases in leaf temperature did not result in further increases in sap flow, which declined slightly. This saturation maybe related to water deficiency and the closing of stomata. This hypothesis is supported by a decrease of soil moisture, stomata conductance, and leaf water potential during September 2015. Our observations demonstrate a strong coupling between leaf temperature and sap flow and suggests the possibility of predicting evapotranspiration fluxes at the ecosystem to regional scales using remote sensing of vegetation temperature.